Insulation analyzer apparatus and method of use

ABSTRACT

An insulation specimen may be analyzed by removing any absorbed charge from the specimen. Test voltage E is applied through system resistance R. I TOT  is sensed at time intervals occurring at selected times multiplied by any three points in a power series while maintaining E constant and R IR  is calculated from the formula: ##EQU1## and displayed. Apparatus consists of a high voltage power supply, current sensing means and specimen terminals for connecting an insulating specimen across the power supply in series with the current sensing means. A voltage comparator is connected across the specimen terminals to sense effective voltage across the specimen. Computation means receives inputs from timing means, the current sensing means and the voltage comparator and is used to calculate insulating current using a formula involving elapsed time measurements of total current through the insulation specimen to calculate insulation current and the insulation resistance of the insulation specimens. Means is employed to adjust the high voltage direct current power supply and display means permits display of insulation current and insulation resistance readings.

The present invention relates to measurement of insulation resistance,and other related properties, of a specimen using a method andpreferably employing a novel insulation analyzer apparatus in accordancewith the present invention.

BACKGROUND OF THE INVENTION

Since close to the beginning of the century, insulation measurementshave been made using techniques which have been highly developed andinstruments built specifically for that purpose such as the Megger® soldby James G. Biddle Company. Such instrumentation requires a high degreeof knowledge and skill on the part of the operator in order to obtainreadings which are reliable and, by following a painstaking procedure,can achieve a high degree of accuracy in measuring insulationresistance. Persons with the skill and patience to practice this methodare becoming relatively unavailable.

In 1958, E. B. Curdts made a technical analysis of insulation testingentitled "Insulation Testing By D-C Methods" which he revised andreprinted in 1964 in Biddle Technical Publication 22T1. In thatpublication, Curdts showed that when d.c. voltage is applied, thecurrent existing in the insulation of a capacitive specimen is alwaysmade up of three components, to wit: geometric capacitance current,i_(g) ; absorption current, i_(a) ; conduction or leakage current,i_(z). These currents will be explained more fully hereafter, but itwill be understood that the current which was measured by allconventional insulation testers was i_(TOT),

    where i.sub.TOT =i.sub.g +i.sub.a +i.sub.c.

Because the i_(TOT) is measured by all conventional insulation testers,various elaborate techniques have been evolved to estimate the value ofi_(c) in the presence of i_(g) and i_(a). Typical examples aretime-resistance tests, step-voltage tests, polarization index anddielectric absorption ratio tests. These tests have been developed to aconsiderable degree of sophistication and achieve excellent results whenapplied by skilled technicians. However, they suffer from thedisadvantages of taking comparatively long periods of time and requiringgreat care and skill in the measurements and their interpretation.

In 22T1, previously referenced E. B. Curdts has described a method ofcalculating i_(c). He shows that since the absorption current is a powerfunction of time, constant n (with a value between 0 and 1.0) is theslope of the straight-line current-time curve plotted on a log-logbasis.

The leakage current i_(c) will represent a deviation from this curve,and may be calculated from ##EQU2## where i₁, i₃.16 and i₁₀ are threevalues of i_(TOT) measured at different times, based on a constant unitof time multiplied by their subscripts, i.e., 1, 3.16 and 10 minutes.

This relationship is true provided that:

a. i_(g) has fallen to a negligible value compared to i_(a) and i_(c) ;which therefore also assumes that

b. i_(g) is negligible at that voltage level when compared to i_(a) andi_(c).

NATURE OF THE PRESENT INVENTION

The present invention involves apparatus for automatically accomplishingthese tests. In particular, the present invention employs an insulationanalyzer apparatus which employs a high voltage direct current powersupply. Current sensing means is employed and specimen terminals areprovided for connecting the insulation specimen across the power supplyin series with the current sensing means. A voltage comparator isconnected across the specimen terminals to sense effective voltageacross the specimen. Timing means is provided. Computation means forrecovering input from the timing means, current sensing means andvoltage comparator in calculating insulation current as a function ofelapsed time measurements of total current through the insulationspecimen to calculate insulation current, and the insulation resistanceof the specimen's insulation. Means are provided to adjust the highvoltage direct current power supply. Display means are also providedpermitting reading of various insulation and other properties of thespecimen.

In accordance with the present invention, the following method ofanalyzing an insulation specimen is provided which may be practicedusing the analyzer apparatus. First, remove any absorbed charge from thespecimen. Next, apply test voltage E through a fixed resistance, R, inseries with E. Then, time the interval from V=0 until V=0.632E acrossthe specimen with capacitance C. As is well known, this is equal to onetime constant for a combination of resistance and capacitance arrangedas shown. Next, using formula t=C×R, where t is time in seconds,calculate C and store result. Next, as an optional step, calculate anddisplay the value of 100CR, start timing and count display from 100CR tozero. In the course of this countdown, maintain E constant and measurei_(TOT) in time increments related in the sequence (t₁, t₂, t₃) 1, 3.16,10, for example at 10CR, 31.6CR and 100CR. Calculate time insulationresistance (R_(IR)) from ##EQU3## and that is where

For a better understanding of the present invention, reference is madeto the drawings in which:

FIG. 1 is a plot of current in microamperes against time in seconds ofi_(g), i_(a), i_(c) and the total of these three currents;

FIG. 2 is a equivalent circuit representing the insulation specimenbroken down into the respective current paths for the three currentcomponents, a possible additional current i_(q), defined below, andshowing the power supply and system resistance; and

FIG. 3 is a block diagram of an insulation analyzer in accordance withthe present invention.

Referring to FIG. 1, plots of the four currents are shown. Thesecurrents may be described as follows:

Geometric Capacitance Current, i_(g)

This is the current due to the capacitance created by the geometricarrangement of electrodes and the dielectric constant(s) of theinsulation(s). ##EQU5## Where E=open circuit voltage of the dc supply;

R=the total internal series resistance;

t=time; and

C=capacitance of the specimen.

Absorption Current, i_(a)

This current results from absorption within the imperfect dielectric,caused by polarizations within the molecular chains of the dielectric.

    i.sub.a =ΔVCDt.sup.-n =At.sup.-n

where

V=the increment of applied voltage

C=capacitance of the specimen

D=a proportionality factor on a per unit basis of applied voltage andthe capacitance of the specimen. This depends on the type of insulationand its condition and temperature.

A=ΔVCD

n=a constant

t=time

Conduction or Leakage Current, i_(c)

This current is the resistive current either through the bulk of theinsulation or over surfaces such as terminal insulator. ##EQU6## whereE_(s) =the applied direct voltage (at the terminals of the specimen)

R_(s) =the insulation resistance of the specimen under test.

Partial Discharge Current, i_(q)

As shown by R. J. Densley in ASTM-STP 660 Chapter 11, an additionalcomponent will occur when the value of E_(s) is above the partialdischarge inception voltage for the specimen. ##EQU7## where f_(i)=discharge repetition rate of the discharges of magnitude, qi, occurringat a discharge site, i. For most purposes i_(q), the partial dischargecurrent can be ignored and the plot i_(t) is equivalent to i_(TOT). Itshould be noted that FIG. 1 is plotted on logarithmic scales.

Referring to FIG. 2, the currents may be visualized as dividing throughthe specimen. As shown, i_(q) is treated as negligible for most purposesand in most cases this is acceptable. In test instrument terms, atypical workable range would be to provide a test supply E with aninternal series resistance of 1M Ω. The capacitive load C of thespecimen might range from 10 pF minimum to 10 μF maximum. Thus, ignoringthe effect of insulation resistance, the system provides a range of timeconstants from RC=10⁻⁵ seconds minimum to 10 seconds maximum. Theseassumptions are illustrative and are not critical to the operation ofthe instrument but permit practical values to be used with the finalspecifications of an instrument designed in each individual case basedupon need.

The method of the present invention in its simple mode of operation canbe specified in the following steps:

Step 1. Conditioning of the specimen. Remove any absorbed chargeremaining by one of three methods: apply reverse dc, reducing inamplitude to zero; or apply ac reducing to zero; or short circuit thespecimen for a significant time. The first two methods are obviouslymore desirable because the time involved is shorter.

Step 2. Apply the test voltage E via R. Measure the time interval from 0until--

    V=0.632E

This time interval gives--

    t (secs)=C×R (C in μF, R in mΩ)

Calculate C, store in memory CR and C.

Step 3. Start Timing. Calculate and display 100CR. Count display down tozero (test finished).

Step 4. Maintaining E constant, measure i_(TOT) in time incrementsrelated in the sequence (t₁, t₂, t₃) 1, 3.16, 10, for example 10CR,31.6CR and 100CR. Calculate true Insulation Resistance (R_(IR)) from##EQU8## and that is where ##EQU9## where R is the resistance 12b shownin FIG. 2.

Step 5. Display and/or print E, R_(IR), C, or any other form that can becalculated from these values, e.g., zero frequency power factor, tan δ,etc. It is also possible to calculate the slope n of the absorptioncurrent i_(a), which may be of interest with some types of insulation.

Operation in a complex mode of operation is also possible whereinmeasurements in the simple mode are repeated at a voltage E, selected tobe below partial discharge inception, and the results stored. Thevoltage is then increased in steps and the value of the R_(IR) at eachvoltage compared with the previous value. The point where a decrease isdetected corresponds to the partial discharge inception. At theconclusion of the test the instrument can read out: C, R_(IR), partialdischarge inception, partial discharge at a predetermined voltage. Thecalculation of partial discharge magnitude is based on measuring Δ_(ic),since Δ_(ic) (representing partial discharge) is based on onecoulomb/sec=1 ampere (by definition). The method of detection of i_(c)requires the use of techniques already developed in conventional partialdischarge measurement, for example, the James G. Biddle Co. Catalog665700-00 partial discharge bridge.

DISCUSSION OF THE MEASUREMENT

Most measurements would be of such a short duration that the delay wouldbe negligible. For example, with a 0.1 μF capacitive specimen, a totalmeasurement time of about 10 secs. is required for the simple mode andfor a full analysis of 10 voltage steps, only 1 min. 40 sec. The worstcase would be a 10 μF specimen where a time of 16.6 minutes is requiredfor a simple measurement. This is probably acceptable, because the testimplies a very unusual measurement such as the largest size of generatoror a very long length of cable. No operator intervention would berequired during the test.

It is quite simple to reduce the timing, either by reducing the initialreading from 10CR to say 5CR (at some loss of accuracy) or by reducingR. In the case of portable field apparatus, the value of R should alsobe selected to permit measurements at a current of less than 5 mA forreasons of safety.

A number of variations of the basic concept are possible.

a. Instead of a constant voltage source for E, it could be replaced by aconstant current generator. This somewhat modifies the calculations butotherwise does not change the concept. The decision therefore depends onpractical and economic considerations.

b. If the initial value of CR is too short for accurate timing with apractical microprocessor clock rate (say 4 MHz) it may be desirable togo back to step 1 and repeat the sequence with the higher value of R.Similarly, if CR is very large, measurements could be automaticallychanged to 50CR total for shorter times at slightly reduced accuracies.

c. If high precision in the measurement of C is required, the initialcalculation which ignores the effect of R_(IR) can be recalculatedtaking R_(IR) into account. (Note that the inaccuracy of the initialdetermination of CR will not affect the value of R_(IR) since it is onlya proportional error.)

d. In the presence of noise it may be advantageous to automaticallyperform iterative measurements and average the result.

DESCRIPTION OF THE MEASUREMENT APPARATUS

Reference is now made to FIG. 3 which illustrates the insulationanalyzer system in schematic form. Typical embodiments of the instrumentare a field portable lightweight instrument with a 5 kV 5 mA maximumsupply for use in the simple mode only. A more elaborate transportableunit for complex mode of operation may be rated at 30 kV 30 mA.

The system consists of a programmable regulated high voltage d.c. powersupply 12 made up of a regulated adjustable voltage source 12a in serieswith a resistor 12b. The voltage of the power supply is regulated to aclose tolerance by an internal feedback loop. The voltage loop is set byreference voltage controlled by a digital signal originating from themicroprocessor via connection 18a.

A specimen 10, which is to be measured to determine its insulationcharacteristics and capacitance, is connected across the power supply 12and in series with the current sensor 14. The current sensor istypically a precision resistor with a value which is small compared withresistance 12b.

An electronic voltage comparator 16 is connected in parallel with thespecimen 10 and the current sensor 14. This compares the voltage acrossthe specimen 10 and current sensor 14 with a reference voltage. When thevoltage reaches 0.632E (where E is the present value in 12a), a digitalsignal is transmitted by connection 16a to the microprocess 18.

The output of the current sensor 14, in the form of an analog voltagedirectly proportional to the sensed current (I_(TOT) in FIG. 2), isconnected to an analog-to-digital converter 21. The output of thisconverter, in digital form, is connected to the microprocessor 18 byconnection 14a.

The microprocessor system 18 and associated memory 20 perform thecontrol, computation and timing function necessary to the measurement.Stored in the read-only portion of the memory 20 is a program whichexecutes the actions listed as steps 1 through step 5 in the preceedingdescription.

The microprocessor 18 and associated integrated circuits and read onlyand random access memories can be any of a wide range of currentlyavailable microprocessor equipment, such as the Radio Corporation ofAmerica's COSMAC Microprocessor Type 1802 Handbook MPG 180C "COSMACMicroprocessor Product Guide". A second example is the Zilog Z80 seriesof microprocessor described in the books "Z80 Microprocessor Programmingand Interfacing", books 1 and 2 by E. A. Nichols, J. C. Nichols and P.R. Rony and the companion book "Microcomputer-Analog Converter Softwareand Hardware Interfacing" by J. A. Titus, C. A. Titus, P. R. Rony, D. G.Larsen, all published by Howard W. Sams & Co., Inc., Indianapolis, IN.The general principles used are similar to those described in"Microcomputers/Microprocessors: Hardware, Software and Applications".J. R. Hilborn, P. M. Tulich, published by Prentice-Hall Inc., EnglewoodClifts, NJ.

The program to implement steps 1 through 5 given above can be in anyform suitable for the microprocessor used.

Alternatives to the use of the microprocessor 18 described above wouldbe a system of hard wired logic or a method of analog measurement withsuitable timing means and a recorder, both of which could be made toachieve the same results by implementing the program Steps 1 through 5in the previous description.

The new instrument has the primary advantage that it can measure with asingle set-up, requiring no special operator skill, the quantity whichgives a true indication of insulation quality. At present the sameinformation can only be obtained by a complex and time consuming seriesof measurements.

The new instrument also provides a means of measuring loss due topartial discharge and establishing partial discharge inception.

These advantages can be obtained with an instrument which is smaller,lighter and costs less than existing techniques.

I claim:
 1. Insulation analyzer apparatus comprising:a programmable highvoltage direct current power supply, current sensing means, specimenterminals for connecting an insulation specimen across said power supplyin series with the current sensing means, a voltage comparator connectedacross the specimen terminals to sense effective voltage across thespecimen, timing means, computation means for receiving inputs from thetiming means, current sensing means and voltage comparator andcalculating insulation current using a formula involving elapsed timemeasurements of total current through the insulation specimen tocalculate insulation current and the insulation resistance of theinsulation specimens; means to adjust the high voltage direct currentpower supply; and display means permitting insulation current andinsulation resistance readings.
 2. The insulation analyzer apparatus ofclaim 1 in which the computation means is a programmable computer. 3.The insulation analyzer apparatus of claim 1 in which test voltage E isheld constant in the high voltage direct current power supply.
 4. Theinsulation analyzer apparatus of claim 1 in which the output current ofthe high voltage direct current power supply is held constant.
 5. Theinsulation analyzer apparatus of claim 1 in which the timing means isset for a predetermined period, and the computation means haspreestablished criteria for adequacy of the timing period results,whereby if the initial values of the product of total specimenresistance and capacitance is too small for convenience in use, thescale is automatically changed by adding a known amount of resistance tothe specimen resistance.
 6. The insulation analyzer apparatus of claim 1in which the timing means is set for a predetermined period, and thecomputation means has preestablished criteria for adequacy of the timingperiod results, whereby if the initial values of the product of totalspecimen resistance and capacitance is too large for convenience in use,the scale is automatically changed by modifying the amount of resistanceadded to the specimen resistance.
 7. A method of analyzing an insulationspecimen comprising:removing any absorbed charge from the specimen;applying test voltage E through system resistance R; sensing I_(TOT) atthree preselected time t₁, t₂ and t₃, while maintaining E constant;calculating and displaying insulation resistance R_(IR) from formula:##EQU10## where ##EQU11## and where t₃ =10t₁ ; t₂ =3.16t₁ and t₁ =anyunit of time arbitarily chosen; and repeating the steps makingrepetitive measurements and averaging the results in order to reduce theeffect of electrical noise.
 8. A method of analyzing an insulationspecimen comprising:removing any absorbed charge from the specimen;applying test voltage E through system resistance R; timing an intervalt until the specimen reaches 0.632E; then calculating C from C=t/R;displaying value of C; starting timing and counting display from 100CRand decrementing by 1CR steps to zero; maintaining E constant, whilemeasuring I_(TOT) at each of the timing intervals at 100CR, 31.6CR and10CR; and calculating and displaying resistance R_(IR) from formula:##EQU12## where ##EQU13##